Lithographic projection apparatus using catoptrics in an optical sensor system, optical arrangement, method of measuring, and device manufacturing method

ABSTRACT

A lithographic projection apparatus according to an embodiment of the invention includes a measurement system configured to determine a position of a target portion of a substrate, using at least one among an optical sensing operation and an optical detecting operation. The position is determined via an optical path that includes at least one catoptrical system arranged to have an imaging function of at least one dioptrical element.

RELATED APPLICATIONS

This application is a continuation-in-part application of Ser. No.09/519,875, filed Mar. 6, 2000, now U.S. Pat. No. 6,674,510, whichclaims priority from European Patent Application No. 99200649.4, filedMar. 8, 1999. This application also is a continuation-in-part of U.S.patent application Ser. No. 10/686,641, filed Oct. 17, 2003, now U.S.Pat. No. 6,882,405, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/519,875 listed above. This application also is acontinuation-in-part of U.S. patent application Ser. No. 10/683,454,filed Oct. 14, 2003, now U.S. Pat. No. 6,924,884, which is acontinuation-in-part of U.S. patent application Ser. No. 09/519,875listed above. The entireties of each of these documents are hereinincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to lithographic projection apparatus andmethods.

BACKGROUND

The term “patterning structure” as here employed should be broadlyinterpreted as referring to any structure or field that may be used toendow an incoming radiation beam with a patterned cross-section,corresponding to a pattern that is to be created in a target portion ofa substrate; the term “light valve” can also be used in this context. Itshould be appreciated that the pattern “displayed” on the patterningstructure may differ substantially from the pattern eventuallytransferred to e.g. a substrate or layer thereof (e.g. where pre-biasingof features, optical proximity correction features, phase and/orpolarization variation techniques, and/or multiple exposure techniquesare used). Generally, such a pattern will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit or other device (see below). A patterningstructure may be reflective and/or transmissive. Examples of patterningstructure include:

A mask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, theundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. An array of grating light valves (GLVs) may also be used in acorresponding manner, where each GLV may include a plurality ofreflective ribbons that can be deformed relative to one another (e.g. byapplication of an electric potential) to form a grating that reflectsincident light as diffracted light. A further alternative embodiment ofa programmable mirror array employs a matrix arrangement of very small(possibly microscopic) mirrors, each of which can be individually tiltedabout an axis by applying a suitable localized electric field, or byemploying piezoelectric actuation means. For example, the mirrors may bematrix-addressable, such that addressed mirrors will reflect an incomingradiation beam in a different direction to unaddressed mirrors; in thismanner, the reflected beam is patterned according to the addressingpattern of the matrix-addressable mirrors. The required matrixaddressing can be performed using suitable electronic means. In both ofthe situations described hereabove, the patterning structure cancomprise one or more programmable mirror arrays. More information onmirror arrays as here referred to can be gleaned, for example, from U.S.Pat. No. 5,296,891 and No. 5,523,193 and PCT patent applications WO98/38597 and WO 98/33096, which documents are incorporated herein byreference. In the case of a programmable mirror array, the supportstructure may be embodied as a frame or table, for example, which may befixed or movable as required.

A programmable LCD panel. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask (or“reticule”) and mask table (or “reticule table”); however, the generalprinciples discussed in such instances should be seen in the broadercontext of the patterning structure as hereabove set forth.

A lithographic apparatus may be used to apply a desired pattern onto asurface (e.g. a target portion of a substrate). Lithographic projectionapparatus can be used, for example, in the manufacture of integratedcircuits (ICs). In such a case, the patterning structure may generate acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g. comprising one or moredies and/or portion(s) thereof) on a substrate (e.g. a wafer of siliconor other semiconductor material) that has been coated with a layer ofradiation-sensitive material (e.g. resist). In general, a single waferwill contain a whole matrix or network of adjacent target portions thatare successively irradiated via the projection system (e.g. one at atime).

Among current apparatus that employ patterning by a mask on a masktable, a distinction can be made between two different types of machine.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally<1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Aprojection beam in a scanning type of apparatus may have the form of aslit with a slit width in the scanning direction. More information withregard to lithographic devices as here described can be gleaned, forexample, from U.S. Pat. No. 6,046,792, which is incorporated herein byreference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (e.g.resist). Prior to this imaging procedure, the substrate may undergovarious other procedures such as priming, resist coating, and/or a softbake. After exposure, the substrate may be subjected to other proceduressuch as a post-exposure bake (PEB), development, a hard bake, and/ormeasurement/inspection of the imaged features. This set of proceduresmay be used as a basis to pattern an individual layer of a device (e.g.an IC). For example, these transfer procedures may result in a patternedlayer of resist on the substrate. One or more pattern processes mayfollow, such as deposition, etching, ion-implantation (doping),metallization, oxidation, chemo-mechanical polishing, etc., all of whichmay be intended to create, modify, or finish an individual layer. Ifseveral layers are required, then the whole procedure, or a variantthereof, may be repeated for each new layer. Eventually, an array ofdevices will be present on the substrate (wafer). These devices are thenseparated from one another by a technique such as dicing or sawing,whence the individual devices can be mounted on a carrier, connected topins, etc. Further information regarding such processes can be obtained,for example, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4.

A substrate as referred to herein may be processed before or afterexposure: for example, in a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist) or a metrologyor inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once (for example, in order tocreate a multi-layer IC), so that the term substrate as used herein mayalso refer to a substrate that already contains multiple processedlayers.

The term “projection system” should be broadly interpreted asencompassing various types of projection system, including refractiveoptics, reflective optics, catadioptric systems, and charged particleoptics, for example. A particular projection system may be selectedbased on factors such as a type of exposure radiation used, anyimmersion fluid(s) or gas-filled areas in the exposure path, whether avacuum is used in all or part of the exposure path, etc. For the sake ofsimplicity, the projection system may hereinafter be referred to as the“lens.” The radiation system may also include components operatingaccording to any of these design types for directing, shaping, reducing,enlarging, patterning, and/or otherwise controlling the projection beamof radiation, and such components may also be referred to below,collectively or singularly, as a “lens.”

Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTApplication No. WO 98/40791, which documents are incorporated herein byreference.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.water) so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. The use of immersiontechniques to increase the effective numerical aperture of projectionsystems is well known in the art.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation or particle flux,including but not limited to ultraviolet radiation (e.g. with awavelength of 365, 248, 193, 157 or 126 nm), EUV (extreme ultra-violetradiation, e.g. having a wavelength in the range 5–20 nm), and X-rays,as well as particle beams (such as ion or electron beams).

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beexplicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, DNA analysis devices, etc. The skilled artisan will appreciatethat, in the context of such alternative applications, any use of theterms “reticle,” “wafer,” or “die” in this text should be considered asbeing replaced by the more general terms “mask,” “substrate,” and“target portion” (or “exposure area”), respectively.

Also herein, aspects of the invention may be described using a referencesystem of orthogonal X, Y and Z directions, and rotation about an axisparallel to an I direction is denoted Ri. Further, unless the contextotherwise requires, the term “vertical” (Z) used herein is intended torefer simply to a direction normal to the substrate or mask surface,rather than implying any particular orientation of the apparatus.

When a lens element is used in air ambient, the optical properties ofthis lens may be different from those in pure nitrogen, helium, orargon. Also, the working ambient may comprise a liquid to improve theoptical performance (such technology is known as immersion lithography).Moreover, other gases or liquids may be considered for differentpurposes (for example in relation to specific properties such as thermalconductance or electrical isolation, etc). Similarly, when the lenselement is placed in vacuum the refraction of light in the lens may alsobe different. Such variations may cause inaccuracies or other problemsin activities such as measurement or device manufacturing.

SUMMARY

A lithographic apparatus according to one embodiment of the inventionincludes a projection system configured to project a patterned beam ofradiation onto a target portion of a substrate and a measurement systemhaving at least one catoptrical system, wherein the measurement systemis configured to determine a position of the target portion in thepatterned beam of radiation, using an optical path that traverses the atleast one catoptrical system.

A lithographic projection apparatus according to another embodiment ofthe invention includes a projection system configured to project apatterned beam of radiation onto a target portion of a substrate and ameasurement system configured to determine a position of the targetportion using at least one among an optical sensing operation and anoptical detecting operation, wherein the measurement system isconfigured to determine the position via an optical path that includesat least one catoptrical system.

A lithographic apparatus according to a further embodiment of theinvention includes a projection system configured to project a patternedbeam of radiation onto a target portion of a substrate and a levelsensor including at least one catoptrical system, wherein the levelsensor is configured to determine a height of a surface of the substrateusing an optical path that includes the at least one catoptrical system.

A device manufacturing method according to a further embodiment of theinvention includes projecting a patterned beam of radiation onto atarget portion of a substrate and using at least one of an opticaldetection action and an optical sensing action to position the targetportion in the patterned beam of radiation via an optical path thatincludes at least one catoptrical system.

An optical analysis system according to a further embodiment of theinvention includes at least one of an optical detector and an opticalsensor configured to determine a position of a target portion of asurface, wherein said at least one of an optical detector and an opticalsensor is configured to determine the position via an optical path thatincludes at least one catoptrical system.

A device manufacturing method according to a further embodiment of theinvention includes projecting a patterned beam of radiation onto atarget portion of a substrate and using a measurement system to positionthe target portion in the patterned beam of radiation via an opticalpath that includes at least one catoptrical system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a view showing how the wafer height may be determined frommeasurements by a level sensor and a Z-interferometer;

FIGS. 3 to 6 are views showing various steps of an off-axis levellingprocedure according to an embodiment of the present invention;

FIG. 7 is a plan view of a substrate table showing sensors and fiducialsused in an off-axis levelling procedure according to an embodiment ofthe present invention;

FIG. 8 is a side view of exposure and measurement stations in a secondembodiment of the invention;

FIG. 9 is a flow diagram illustrating various steps of the measurementprocess carried out at a measurement station of the second embodiment ofthe invention;

FIG. 10 is a flow diagram illustrating various tasks of an exposureprocess carried out at an exposure station of the second embodiment ofthe present invention;

FIG. 11 is a diagram illustrating the scan pattern usable to measure aheight map in a method according to an embodiment of the presentinvention;

FIG. 12 is a diagram illustrating an alternative scan pattern usable tomeasure the height map of the present invention;

FIG. 13 is a diagram illustrating a global level contour process in thesecond embodiment of the present invention;

FIG. 14 and its sub-Figures A to G illustrate the structure andoperation of a level sensor according to an embodiment of the invention;

FIG. 15 is a graph showing detector output vs. substrate table positionfor a capture spot of the level sensor of FIG. 14;

FIG. 15A is a diagram showing the arrangements of detector segments forthe capture spot of the level sensor of FIG. 14;

FIGS. 16 and 17 are diagrams illustrating a confidence sensor accordingto an embodiment of the invention;

FIG. 18 is a diagram of a beam splitter usable in the confidence sensorof FIGS. 16 and 17;

FIG. 19 is a diagram used to explain a Z-interferometer calibrationprocedure according to an embodiment of the invention;

FIG. 20 is a diagram illustrating the notation used in describing anexposure trajectory optimization procedure according to a thirdembodiment of the invention; and

FIG. 21 depicts a lithographic projection apparatus according to a fifthembodiment of the invention;

FIG. 22 depicts a dioptrical element; and

FIG. 23 depicts a catoptrical system according to an embodiment of thepresent invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

Embodiments of the invention include, for example, a lithographicapparatus and a device manufacturing method which require substantiallyno ambient-related corrections of the optical system.

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

A radiation system configured to supply (e.g. having structure capableof supplying) a projection beam of radiation (e.g. UV or EUV radiation).In this particular example, the radiation system RS comprises aradiation source LA, a beam expander Ex, and an illumination systemincluding an integrator IN and condensing optics CO (and possibly alsoan adjusting structure AM for setting an illumination node);

A support structure configured to support a patterning structure capableof patterning the projection beam. In this example, a first object table(mask table) MT is provided with a mask holder for holding a mask MA(e.g. a reticle), and is connected to a first positioning structure foraccurately positioning the mask with respect to item PL;

A second object table (substrate table) configured to hold a substrate.In this example, substrate table WTa is provided with a substrate holderfor holding a substrate W (e.g. a resist-coated semiconductor wafer),and is connected to a second positioning structure for accuratelypositioning the substrate with respect to item PL and (e.g.interferometric) measurement structure IF, which is configured toaccurately indicate the position of the substrate and/or substrate tablewith respect to lens PL;

A third object table (substrate or wafer table) WTb provided with asubstrate holder configured to hold a substrate W (e.g. a resist-coatedsilicon wafer), and connected to a third positioning structureconfigured to accurately position the substrate with respect to item PLand another (e.g. interferometric) measurement structure IF, which isconfigured to accurately indicate the position of the substrate and/orsubstrate table with respect to lens PL; and

A projection system (“lens”) configured to project the patterned beam.In this example, projection system PL (e.g. a refractive lens group, acatadioptric or catoptric system, an array of field deflectors, and/or amirror system) is configured to image an irradiated portion of the maskMA onto a target portion C (e.g. comprising one or more dies and/orportion(s) thereof) of a substrate W held on a substrate table WTa orWTb (e.g. at an exposure station). Alternatively, the projection systemmay project images of secondary sources for which the elements of aprogrammable patterning structure may act as shutters. The projectionsystem may also include a microlens array (MLA), e.g. to form thesecondary sources and to project microspots onto the substrate.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning structure, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. a mercury lamp, an excimer laser, an electron gun, alaser-produced plasma source or discharge plasma source, or an undulatorprovided around the path of an electron beam in a storage ring orsynchrotron) produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed a conditioning structure or field, such as a beam shapingoptics Ex, for example. The illuminator IL may comprise an adjustingstructure or field AM for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in the beam, which may affect the angulardistribution of the radiation energy delivered by the projection beamat, for example, the substrate. In addition, the apparatus willgenerally comprise various other components, such as an integrator INand a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable direction mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed (alternatively, having been selectivelyreflected by) the mask MA, the beam PB passes through the lens PL, whichfocuses the beam PB onto a target portion C of the substrate W. With theaid of the second or third positioning structure (and correspondinginterferometric measuring structure IF), the substrate table WTa or WTbcan be moved accurately, e.g. so as to position different targetportions C in the path of the beam PB. Similarly, the first positioningstructure can be used to accurately position the mask MA with respect tothe path of the beam PB, e.g. after mechanical retrieval of the mask MAfrom a mask library, or during a scan. In general, movement of theobject tables MT, WTa, WTb will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. However, inthe case of a wafer stepper (as opposed to a step-and-scan apparatus)the mask table MT may just be connected to a short stroke actuator (e.g.to make fine adjustments in mask position and/or orientation), or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The second and third positioning structures may be constructed so as tobe able to position their respective substrate tables WTa, WTb over arange encompassing both a exposure station under projection system PLand a measurement station under measurement system MS. Alternatively,the second and third positioning structures may be replaced by separateexposure station and measurement station positioning systems forpositioning a substrate table in the respective exposure stations and atable exchange means for exchanging the substrate tables between the twopositioning systems. Suitable positioning systems are described, interalia, in WO 98/28665 and WO 98/40791 mentioned above. It should be notedthat a lithography apparatus may have multiple exposure stations and/ormultiple measurement stations, that the numbers of measurement andexposure stations may be different than each other, and that the totalnumber of stations need not equal the number of substrate tables.Indeed, the principle of separate exposure and measurement stations maybe employed even with a single substrate table.

The depicted apparatus can be used in several different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at once (i.e. in a single “flash”)onto a target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB. In step mode, a maximum size of the exposurefield may limit the size of the target portion imaged in a single staticexposure;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image. Concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). The velocity and/or direction of the substrate table WTrelative to the mask table MT may be determined by magnification,demagnification (reduction), and/or image reversal characteristics ofthe projection system PL. In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution. Inscan mode, a maximum size of the exposure field may limit the width (inthe non-scanning direction) of the target portion exposed in a singledynamic exposure, whereas the length of the scanning motion maydetermine the height (in the scanning direction) of the target portionexposed;

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning structure, and the substrate table WTis moved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning structureis updated as required after each movement of the substrate table WT orin between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning structure, such as a programmable mirror arrayof a type as referred to above.

Combinations of and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

Until very recently, lithographic apparatus contained a single masktable and a single substrate table. However, machines are now becomingavailable in which there are at least two independently moveablesubstrate tables; see, for example, the multi-stage apparatus describedin International Patent Applications WO98/28665 and WO98/40791. Thebasic operating principle behind such multi-stage apparatus is that,while a first substrate table is at an exposure position underneath theprojection system for exposure of a first substrate located on thattable, a second substrate table can run to a loading position, dischargea previously exposed substrate, pick up a new substrate, perform someinitial measurements on the new substrate and then stand ready totransfer the new substrate to the exposure position underneath theprojection system as soon as exposure of the first substrate iscompleted; the cycle then repeats. In this manner it is possible toincrease substantially the machine throughput, which in turn may improvethe cost of ownership of the machine. It should be understood that thesame principle could be used with just one substrate table which ismoved between exposure and measurement position.

Mainstream lithographic apparatus comprise an optical sensor system forsensing alignment and leveling. Such an optical sensor system comprisesdioptrical components and, in some cases, also diffractive components.Typically, light is generated by an optical source and guided throughthe optical sensor system for imaging on the substrate. The dioptricalcomponents are designed and constructed in such a way to allow sensingof signals, generated on the dedicated structure, and for these signalsto have sufficient quality to derive from them parameters related to thespecific measurement by the sensor system.

It is a problem of the prior art that the ambient in which the opticalsystem is operated may influence its imaging properties. For example,the refractive power of a lens may vary with the ambient and thus theimaging properties of the optical system may vary with the ambient.

Thus, if for example, a lithographic apparatus is to be used duringdevice manufacturing in a specific ambient such as nitrogen, differentfrom the ambient in which the lithographic apparatus is tested andcalibrated, it may be desired or necessary to apply a correction duringcalibration to compensate for deviations when going to the manufacturingambient. Such a correction may consist of a mechanical adaptation of aposition of the optical system relative to the projection plane for theimage or the addition of an extra optical component. This test andcalibration procedure is not very efficient and, obviously, requireschecking during operation of the lithographic apparatus.

An important factor influencing the imaging quality of a lithographicapparatus may be the accuracy with which the mask image is focused onthe substrate. In practice, since the scope for adjusting the positionof the focal plane of the projection system PL is limited and the depthof focus of that system is small, it may be desirable or necessary forthe exposure area of the wafer (substrate) to be positioned precisely inthe focal plane of the projection system PL. To do this, it may benecessary to know both the position of the focal plane of the projectionsystem PL and the position of the top surface of the wafer.

Wafers are polished to a very high degree of flatness, but neverthelessdeviation of the wafer surface from perfect flatness (referred to as“unflatness”) of sufficient magnitude noticeably to affect focusaccuracy can occur. Unflatness may be caused, for example, by variationsin wafer thickness, distortion of the shape of the wafer, orcontaminants on the wafer holder. The presence of structures due toprevious process steps may also significantly affect the wafer height orflatness. In at least some embodiments of the present invention, thecause of unflatness is largely irrelevant; only the height of the topsurface of the wafer is considered. Unless the context otherwiserequires, references below to “the wafer surface” refer to the topsurface of the wafer onto which (or onto a layer upon which) will beprojected the mask image.

In a method according to an embodiment of the invention, after loading awafer onto the substrate table, the height of the wafer surfaceZ_(wafer) relative to a physical reference surface of the substratetable is mapped. This process is carried out at the measurement stationusing a first sensor (referred to as the “level sensor”) which measuresthe vertical (Z) position of the physical reference surface and thevertical position of the wafer surface, Z_(LS), at a plurality ofpoints, and a second sensor (for example, a Z-interferometer) whichsimultaneously measures the vertical position of the substrate table,Z_(IF) e.g. at the same points. As shown in FIG. 2, the wafer surfaceheight is determined as Z_(Wafer)=Z_(LS)−Z_(IF). The substrate tablecarrying the wafer is then transferred to the exposure station and thevertical position of the physical reference surface is again determined.The height map is then referred to in positioning the wafer at thecorrect vertical position during the exposure process. This procedure isdescribed in more detail below with reference to FIGS. 3 to 6.

As shown in FIG. 3, first the substrate table is moved so that aphysical reference surface fixed to the substrate table is underneaththe level sensor LS. The physical reference surface may be anyconvenient surface whose position in X, Y and Z on the substrate tablewill not change during processing of a wafer in the lithographicapparatus and, perhaps most importantly, in the transfer of thesubstrate table between measurement and exposure stations. The physicalreference surface may be part of a fiducial containing other alignmentmarkers. It may be desirable for this surface to have such properties asallow its vertical position to be measured by the same sensor asmeasures the vertical position of the wafer surface. In one embodimentof the invention, the physical reference surface is a reflective surfacein a fiducial in which is inset a so-called transmission image sensor(TIS). The TIS is described further below.

The level sensor may be, for example, an optical sensor such as thatdescribed in U.S. Pat. No. 5,191,200, referred to therein as a focuserror detection system); alternatively, a pneumatic or capacitive sensor(for example) is conceivable. One form of a sensor making use of Moirépatterns formed between the image of a projection grating reflected bythe wafer surface and a fixed detection grating is described below inrelation to a second embodiment of the invention. The level sensor maymeasure the vertical position of a plurality of positions simultaneouslyand for each may measure the average height of a small area, e.g. soaveraging out unflatnesses of high spatial frequencies.

Simultaneously with the measurement of the vertical position of aphysical reference surface by the level sensor LS, the vertical positionof the substrate table may be measured using the Z-interferometer,Z_(IF). The Z-interferometer may, for example, be part of a three-,five-, or six-axis interferometric metrology system such as thatdescribed in e.g. WO 99/28790 or WO 99/32940. It may be desirable forthe Z-interferometer system to measure the vertical position of thesubstrate table at a point having the same position in the XY plane asthe calibrated measurement position of the level sensor LS. This may bedone by measuring the vertical position of two opposite sides of thesubstrate table WT at points in line with the measurement position ofthe level sensor and interpolating/modelling between them. Such anapproach may ensure that, in the event that the wafer table is tiltedout of the XY plane, the Z-interferometer measurement correctlyindicates the vertical position of the substrate table under the levelsensor.

It may be desirable for this process to be repeated with at least asecond physical reference surface spaced apart, e.g. diagonally, fromthe first physical reference surface. Height measurements from two ormore positions can then be used to define a reference plane.

The simultaneous measurement of the vertical position of one or morephysical reference surfaces and the vertical position of the substratetable establishes a point or points determining the reference planerelative to which the wafer height is to be mapped. A Z-interferometerof the type mentioned above is effectively a displacement sensor ratherthan an absolute sensor, and so may require zeroing, but may alsoprovide a highly linear position measurement over a wide range. On theother hand, suitable level sensors, e.g. those mentioned above, mayprovide an absolute position measurement with respect to an externallydefined reference plane (i.e. nominal zero) but over a smaller range.Where such sensors are used, it is convenient to move the substratetable vertically under the level sensor until the physical referencesurface(s) is (are) positioned at a nominal zero in the middle of themeasurement range of the level sensor and to read out the currentinterferometer Z value.

One or more of these measurements on physical reference surfaces willestablish the reference plane for the height mapping. TheZ-interferometer may then be zeroed with reference to the referenceplane. In this way the reference plane is related to the physicalsurface on the substrate table, and the Z_(Wafer) height map may be madeindependent of the initial zero position of the Z-interferometer at themeasurement station and other local factors such as any unflatness inthe base plate over which the substrate table is moved. Additionally,the height map may be made independent of any drift in the zero positionof the level sensor.

As illustrated in FIG. 4, once the reference plane has been established,the substrate table is moved so that the wafer surface is scannedunderneath the level sensor to make the height map. The verticalposition of the wafer surface and the vertical position of the substratetable are measured at a plurality of points of known XY position andsubtracted from each other to give the wafer height at the known XYpositions. These wafer height values form the wafer height map which canbe recorded in any suitable form. For example, the wafer height valuesand XY coordinates may be stored together in so-called indivisiblepairs. Alternatively, the points at which wafer height values are takenmay be predetermined, e.g. by scanning the wafer along a predeterminedpath at a predetermined speed and making measurements at predeterminedintervals, so that a simple list or array of height values (optionallytogether with a small number of parameters defining the measurementpattern and/or a starting point) may suffice to define the height map.

The motion of the substrate table during the height mapping scan islargely only in the XY plane. However, if the level sensor LS is of atype which only gives a reliable zero reading, the substrate table mayalso be moved vertically to keep the wafer surface at the zero positionof the level sensor. The wafer height may then be essentially derivedfrom the Z movements of the substrate table, as measured by theZ-interferometer, performed to maintain a zero readout from the levelsensor.

However, it may be desirable to use a level sensor that has anappreciable measurement range over which its output is linearly relatedto wafer height, or can be linearized. Such measurement range ideallyencompasses the maximum expected, or permissible, variation in waferheight. With such a sensor, the need for vertical movement of thesubstrate table during the scan may be reduced or eliminated and thescan can be completed faster, since the scan speed is then limited bythe sensor response time rather than by the ability of the short-strokesubstrate table to track the contour of the wafer in three dimensions.Also, a sensor with an appreciable linear range can allow the heights ata plurality of positions (e.g. an array of spots) to be measuredsimultaneously.

Next, the wafer table is moved to the exposure station and, as shown inFIG. 5, the (physical) reference surface is positioned under theprojection lens so as to allow a measurement of its vertical positionrelative to the focal plane of the projection lens. In one embodiment,this positioning is achieved using one or more transmission imagesensors (described below) whose detector is physically connected to thereference surface used in the earlier measurements. The transmissionimage sensor(s) can determine the vertical focus position of theprojected image from the mask under the projection lens. Armed with thismeasurement, the reference plane can be related to the focal plane ofthe projection lens, and a path for the substrate table inthree-dimensions which keeps the wafer surface in optimum focus can bedetermined.

One method by which this can be done is to calculate Z, Rx and Rysetpoints for a series of points along the scan path. The setpoints maybe determined using a least squares method so as to minimize thedifference between the wafer map data and the focus plane of theexposure slit image. For ease of calculation, the relative motion of theexposure slit image and wafer can be expressed as the slit movingrelative to a static wafer. The least squares criterion can then beexpressed as, for each time t, finding the values Z(t), Rx(t) and Ry(t)which give a minimum value of:

$\begin{matrix}{{{LSQ}(t)} = {{\frac{1}{s} \cdot \frac{1}{W}}{\int_{{- s}/2}^{s/2}{\int_{{- W}/2}^{W/2}{\left\lbrack {{w\left( {x,y} \right)} - \left( {{Z(t)} + {{x \cdot R}\;{y(t)}} - {{y \cdot R}\;{x(t)}}} \right)} \right\rbrack^{2}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & (1)\end{matrix}$

where w(x,y) is the wafer height map and the exposure slit image is arectangular plane of width s in the scanning direction and length Wperpendicular to the scanning direction with its position defined byz(t), Rx(t) and Ry(t). The setpoints and the wafer trajectory can beexpressed as functions of either Y (position in the scanning direction)or t (time) since these are related by Y=y₀+v.t, where y₀ is thestarting position and v is the scanning speed.

As mentioned above, the physical reference surface(s) is (are)preferably a surface in which a transmission image sensor (TIS) isinset. As shown in FIG. 7, two sensors TIS1 and TIS2 are mounted on afiducial plate mounted to the top surface of the substrate table (WT,WTa or WTb), at diagonally opposite positions outside the area coveredby the wafer W. The fiducial plate may be made of a highly stablematerial with a very low coefficient of thermal expansion (e.g. Invar)and may have a flat reflective upper surface which may carry markersused in alignment processes.

TIS1 and TIS2 are sensors used to determine directly the vertical (andhorizontal) position of the aerial image of the projection lens. Theycomprise apertures in the respective surface close behind which isplaced a photodetector sensitive to the radiation used for the exposureprocess. To determine the position of the focal plane, the projectionlens projects into space an image of a TIS pattern TIS-M provided on themask MA and having contrasting light and dark regions. The substratestage is then scanned horizontally (in one or preferably two directions)and vertically so that the aperture of the TIS passes through the spacewhere the aerial image is expected to be. As the TIS aperture passesthrough the light and dark portions of the image of the TIS pattern, theoutput of the photodetector will fluctuate. The vertical level at whichthe rate of change of amplitude of the photodetector output is highestindicates the level at which the image of TIS pattern has the greatestcontrast and hence indicates the plane of optimum focus. An example of aTIS of this type is described in greater detail in U.S. Pat. No.4,540,277. Instead of the TIS, a Reflection Image Sensor (RIS) such asthat described in U.S. Pat. No. 5,144,363 may also be used.

Using the surface of the TIS as the physical reference surface has thepotential advantage that the TIS measurement may directly relate thereference plane used for the height map to the focal plane of theprojection lens, and so the height map can be employed directly to giveheight corrections for the wafer stage during the exposure process. Thisis illustrated in FIG. 6, which shows the substrate table WT aspositioned under the control of the Z-interferometer at a heightdetermined by the height map so that the wafer surface is at the correctposition under the projection lens PL.

The TIS surface may additionally carry reference markers whose positionis detected using a through-the-lens (TTL) alignment system to align thesubstrate table to the mask. Such an alignment system is described inEuropean Patent Publication No. EP 0,467,445 A, for example. Alignmentof individual exposure areas can also be carried out, or may be obviatedby an alignment procedure carried out at the measurement stage to alignthe exposure areas to the reference markers on the wafer stage. Such aprocedure is described in European Patent Publication No. EP 0 906 590 Afor example.

It will be appreciated that the mask image projected by the projectionsystem PL in a production process, in both step-and-repeat andstep-and-scan modes, is not a single point but extends over asignificant area in the XY plane. Since the wafer height may varysignificantly over this area it is desirable to optimize the focus overthe whole of the projection area, rather than just at a single point. Inembodiments of the present invention, this can be achieved bycontrolling not only the vertical position of the substrate table WT,but also its tilt about the X and Y axes (Rx, Ry). With knowledge of thelocation and extent of the intended exposure areas, the height mapgenerated by the present invention can be used to calculate in advanceoptimum Z, Rx and Ry position setpoints for the substrate table for eachexposure. This avoids the time required for levelling in known apparatusthat only measure wafer height when the wafer is positioned under theprojection lens and hence increases throughput. The optimum Z, Rx and Rysetpoints may be calculated by various known mathematical techniques,for example using an iterative process to minimize defocus (defined asthe distance between the wafer surface and the ideal focal plane), LSQ(t), integrated over the exposure area.

A further advantage is possible in the step-and-scan mode. In this mode,the projection lens projects an image of only part of the mask patternonto a corresponding part of the exposure area. The mask and substrateare then scanned in synchronism through the object and image focalplanes of the projection system PL so that the entire mask pattern isimaged onto the whole exposure area. Although in practice the projectionlens is held stationary and the mask and substrate are moved, it isoften convenient to consider this process in terms of an image slitmoving over the wafer surface. With the height map determined in advanceby the present invention, it is possible to calculate a sequence of Z,Rx and Ry setpoints matched to the XY scan path (usually, scanning takesplace in only one direction, e.g. Y). The sequence of setpoints can beoptimized according to additional criteria, e.g. to minimize verticalaccelerations or tilt motions that might reduce throughput or induceundesirable vibrations. Given a sequence of spaced-apart setpoints, ascanning trajectory for the exposure can be calculated using apolynomial or spline fitting procedure.

Whilst the present invention aims to position the wafer at the optimumposition in Z, Rx and Ry for a given exposure, the variations in wafersurface height over the exposure area may be such that the wafer cannotbe positioned to give adequate focus over the entire area. Suchso-called focus spots can result in an exposure failure. However, withthe present invention such failures can be predicted in advance andremedial action can be taken. For example, the wafer may be stripped andrecoated without the detrimental effect of further processing a badlyexposed wafer. Alternatively, if the predicted failure affects only oneor a few devices on the wafer whilst others will be acceptable,throughput may be enhanced by skipping exposures that can be predictedin advance to result in defective devices.

A further advantage of focus-spot detection can be derived from analysisof height maps taken. When large deviations from a global wafer planeare present in a wafer height map, this could indicate focus spots dueto substrate unflatness or process influences. Comparing wafer heightmaps from several wafers can indicate focus spots due to contaminationor unflatness of the substrate table. When focus spots appear atidentical or near-identical positions for different wafers, this is mostlikely caused by substrate holder contamination (so-called“chuck-spots”). From one wafer height map, one can also compare theheight map (topology) from repeated exposure areas (dies). If largedifferences occur at certain dies with respect to an average height map,one can suspect focus spots due to either wafer processing or thesubstrate table. Instead of comparing wafer height maps, the samecomparisons can also be done on the derived exposure paths per die, oron the defocus parameters MA, MSD or Moving Focus explained below. Whena certain die or wafer deviates much from an average exposure path ordefocus parameters, focus spots can also be detected.

All of the above mentioned analysis can be done before a wafer isexposed, and remedial action, such as wafer rejection (processinginfluences) or substrate holder cleaning (chuck spots), can be taken.With these methods, focus spots can be localised to the size of themeasurement spot of the level sensor 10. This implies a much higherresolution than previous methods of focus spot detection.

EMBODIMENT 2

A second embodiment of the present invention is shown in FIG. 8, whichshows only the exposure and measurement stations and only componentsrelevant to the discussion below. The second embodiment utilizes thelevelling principle of the present invention described above, togetherwith certain refinements described below.

At the exposure station, to the left of FIG. 8, the projection lens PLis shown mounted to metrology frame MF and projecting an image of TISmarker TIS-M on mask MA onto the sensor TIS mounted to wafer table WT.The metrology frame is isolated from the transmission of vibrations fromother parts of the apparatus and has mounted on it only passivecomponents used for fine metrology and alignment sensing. The wholemetrology frame may be made of a material with a very low coefficient ofthermal expansion, such as Invar, so that it provides a very stableplatform for the most sensitive measuring devices of the apparatus. Thecomponents mounted on the metrology frame MF include mirrors 34 and 35to which the measurement beams of the Z-interferometer Z_(IF) aredirected by 45°-mirrors 31 mounted on the sides of the wafer table WT.To ensure that the Z position of the substrate table can be measuredthroughout its range of movement in X, the mirrors 34, 35 have acorrespondingly large extent in the X direction. To ensure the Zposition can be measured throughout the range of Y-movement, the mirrors31 cover the whole length of the wafer table. Also mounted to themetrology frame MF are the beam generating and receiving parts 21 a, 22a of a confidence sensor 20 a described further below.

At the measurement station(on the right in FIG. 8), the same metrologyframe MF carries mirrors 33 and 32 which serve the same function as themirrors 34, 35 at the exposure station; again mirrors 32, 33 will have alarge extent in the X direction to accommodate the required range ofmovement of the substrate table WT which is just as large as that at theexposure station. Level sensor 10, comprising beam generating part 11and detection part 12, is also mounted on the metrology frame MF.Additionally, a confidence sensor 20 b, essentially the same asconfidence sensor 20 a at the exposure position, is provided. Othermeasurement devices, for example an alignment module, can also beprovided.

As discussed above, the use of the physical reference surface (again inthis embodiment this is provided by the upper surface of the TIS)relates the wafer height map to the wafer stage and makes it independentof the zero positions of the two Z-interferometers and certain localfactors such as unflatness of the base plate (stone) BP over which thewafer tables move. However, since the wafer height map is generatedusing the Z-interferometer at the measurement station and the substratetable position is controlled at the exposure station using a differentZ-interferometer provided there, any differences as a function of XYposition between the two Z-interferometers can affect the accuracy withwhich the wafer surface is positioned in the focal plane. The principalcause of these variations in an interferometer system of the type usedin the present invention is unflatness of the mirrors 32, 33, 34, 35.The 45° mirrors 31 are attached to the wafer table WT and travel with itas it swaps between exposure and measurement stations. Any unflatness ofthese mirrors therefore has largely the same effect on positioning atthe exposure station as at the measurement station, and largely cancelsout. However, the mirrors 32, 33, 34 and 35 mounted on the metrologyframe MF stay with their respective interferometers and so anydifferences in the surface contours of the corresponding pairs 32, 34and 33, 35 can adversely affect the vertical positioning accuracy of thesubstrate table WT.

The confidence sensors 20 a and 20 b are used at initial set-up of theapparatus, and periodically as required thereafter, to calibrate thedifferences between the Z-interferometers at the measurement andexposure stations. The confidence sensors are sensors capable ofmeasuring the vertical position of the upper surface of the wafer at oneor more points as the substrate table is scanned underneath it.Confidence sensors 20 a and 20 b can be similar in design to levelsensor 10 but need not be; since they are used only at setup (and forinfrequent recalibration) and with a reference wafer rather thanproduction wafers, the design criteria are less onerous and advantagecan be taken of this to design a simpler sensor. Conversely, theexistence of the projection lens PL at the exposure station willrestrict the physical locations available for the confidence sensor atthat station, and this also needs to be taken into account in design orselection of each confidence sensor. High accuracy is required of theconfidence sensors since the calibration they are used for will affectthe quality of every exposure.

In the calibration process using the confidence sensor(s), a referencewafer is loaded onto the substrate table. The reference wafer ispreferably a bare silicon wafer. There is no requirement for it to beany flatter than a normal bare Si wafer but its surface finish (in termsof reflectivity) is preferably optimized for the confidence sensors. Ina preferred embodiment of the invention the reference wafer ispreferably polished to maximize its reflectivity and minimizeunflatness.

In the calibration procedure, a partial height map of the referencewafer (as usual referenced to the physical reference surface) isgenerated at the measurement station using the confidence sensor 20 binstead of the level sensor 10. This is done in the same manner as withthe level sensor 10: the physical reference surface (TIS) is positionedat the zero point of the confidence sensor to zero the Z-interferometer,the wafer is then scanned under the confidence sensor, and the heightmap is generated from the difference between the confidence sensor andZ-interferometer readings. A height map is also generated at theexposure station using the confidence sensor 20 a at the same points asthe measurement station height map. For this calibration, the heightmaps need not be a complete scan of the wafer; they need only coverstrips corresponding to the movement of the Z-interferometer beam overthe mirrors 32–35. (The order in which the maps are created is notimportant, provided the wafer is stable on the substrate table whilstboth are done.)

Since the height maps represent the same wafer, any differences betweenthem will be caused by differences between the measurement systems usedto create them. The two confidence sensors are static, so their effectson the height maps will not be position-dependent and can be eliminatedby normalizing the two height maps and/or subtracting static offsets.Any remaining differences will be position-dependent, and the two heightmaps can be subtracted from one another to generate correction tables(mirror maps) that relates the exposure station Z-interferometer to themeasurement station Z-interferometer. These correction tables can beattributed to the differences between the mirrors 33, 35 and 32, 34attached to the metrology frame MF and can be applied to the waferheight maps generated in a production process, or used to correct one ofthe Z-interferometers used to generate the map or to position thesubstrate table during the exposure. Depending on the preciseconstruction of the Z-interferometers, particularly the metrology framemirrors and substrate table mirrors, the differences in Z positioncaused by the unflatnesses of the mirrors in each interferometer systemmay also be tilt dependent in one or more degrees of freedom (Rx, Ry,Rz). To eliminate this tilt dependence it may be desirable or necessaryto use the confidence sensors to create several height maps with thewafer stage at various different tilts, from which a number of differentcorrection tables (mirror maps) can be derived, as desired or necessary.

Having described the principle of the off-axis levelling procedure, nowwill be described some further refinements to it that are employed inthe second embodiment, as well as how it is integrated into theproduction process. FIGS. 9 and 10 are referred to and respectively showthe steps carried out at the measurement station and at the exposurestation. In a lithography apparatus using two wafer tables, one tablewill be going through the steps of FIG. 9 whilst a second simultaneouslygoes through the steps of FIG. 10 before they are swapped. In thedescription below, the “life” of a single wafer is followed frommeasurement station (FIG. 9) to exposure station (FIG. 10) and back.

Starting at step S1 in FIG. 9, a wafer coated with a photosensitiveresist is loaded on to the substrate table WT. (Note that this maygenerally take place at a loading station separate from the measurementstation at which the substrate table is out of range of theinterferometer system IF.) The wafer table is moved into the capturerange(s) of one or more position sensitive devices (PSDs) so that aninitial coarse zeroing of the interferometric metrology system can beperformed, step S2. After the initial coarse zeroing, the fineinitialization/zeroing of the interferometric system follows in steps S3and S4. These two steps contain the level sensor measurements (denoted“LS”) on the (two or more) physical reference surfaces, which willdefine the reference plane (fixed to the wafer table) with respect towhich the wafer height map is measured. Also, two alignment measurements(denoted “AA”) are done on markers located on the same physicalreference surfaces, so as to define the horizontal reference positionsfixed to the wafer table. These measurements in S3 and S4 effectivelyzero the interferometric system in all degrees of freedom.

The next step in the levelling procedure is step S5, referred to as theglobal level contour (GLC). In this step, which is described furtherbelow, a wafer capture and an initial scan of the wafer with the levelsensor is made to determine its overall height and tilt as well as itsapproximate height at the points where the subsequent detailed scan willmove onto or off the wafer. This information enables the substrate tabletrajectory for the wafer height map scan to be defined.

In step S6, a global alignment of the wafer is done. At least twoalignment markers on the wafer are measured (W1 and W2), meaning thattheir XY position is determined with respect to the reference markers onthe TIS fiducials. This determines to what extent the wafer ishorizontally rotated (Rz) with respect to the scan direction (y), and isdone to be able to correct the wafer rotation such that the wafer heightmap scans are done parallel to the exposure area axis (i.e. “goingstraight over the exposure areas”).

After that, the levelling procedure may continue with such measurementsas may be desirable or necessary for a process dependent correction(PDC). A process dependent correction may be desirable or necessary withsome forms of level sensor, and will now be explained.

The wafer height map must be taken each time a wafer is exposed. If awafer has already been subjected to one or more process steps, thesurface layer will no longer be pure polished silicon and there may alsobe structures or topology representing the features already created onthe wafer. Different surface layers and structures can affect the levelsensor readings and in particular can alter its linearity. If the levelsensor is optical, these effects may, for example, be due to diffractioneffects caused by the surface structure or by wavelength dependence inthe surface reflectivity, and cannot always be predicted. To determinethe required process dependent correction, an exposure area or die isscanned under the level sensor with the substrate table WT set toseveral different vertical positions spanning the linear or linearizedrange of the level sensor 10. The wafer height, i.e. the physicaldistance between the wafer surface and the reference plane, should notchange with the vertical position of the substrate table; it is obtainedby subtracting the measurements of the level sensor andZ-interferometer: Z_(WAFER)=Z_(LS)−Z_(IF). Therefore if the determinedvalue of Z_(WAFER) does change with vertical position of the substratetable this indicates that either or both the level sensor orZ-interferometer are not linear or not equally scaled. TheZ-interferometer is deemed to be linear since it looks at the mirrors onthe wafer table and metrology frame; and in fact is linear to a greaterextent than the required accuracy for the wafer height map, at leastonce the correction determined by the use of the confidence sensor isapplied. Therefore, any differences in the wafer height values areassumed to result from non-linearity or mis-scaling of the level sensor.They, and the knowledge of at which level sensor readings they wereobserved, can be used to correct the output of the level sensor. It hasbeen found in a presently preferred embodiment of the level sensor thata simple gain correction is sufficient, but a more complex correctionmay be required for other sensors.

If the wafer to be processed has exposure areas on it that have beensubjected to different processes, then a process-dependent correction isdetermined for each different type of exposure area on the wafer.Conversely, if a batch of wafers having exposure areas that haveundergone the same or similar processes are to be exposed, it maysuffice to measure the process-dependent correction for each type ofexposure area once per batch. That correction can then be applied eachtime that type of exposure area is height-mapped in the batch.

In many IC fabs, the photosensitive resist is applied to the waferimmediately before it is loaded into the lithography apparatus. Forthis, and other, reasons, the wafer may be at a different temperaturethan the substrate table when it is loaded and clamped in place. Whenthe wafer cools (or warms) to the same temperature as the substratetable, thermal stresses can be set up because the wafer is clamped veryrigidly using vacuum suction. These may result in undesirable distortionof the wafer. Thermal equilibrium is likely to have been reached by thetime the steps S2 to S7 have been completed. Therefore, at step S8, thevacuum clamping the wafer to the substrate table is released, to allowthe thermal stresses in the wafer to relax, and then reapplied. Thisrelaxation may cause small shifts in the position and/or tilt of thewafer but these are acceptable since steps S2 to S4 are independent ofthe wafer and S5 and S6 are only coarse measurements. Any shift in thewafer position at this stage does not affect the process-dependentcorrection since that is a calibration of the level sensor rather than ameasurement of the wafer.

After the vacuum has been reapplied, and from here on it is not releasedagain until the exposure process is finished, the Z-map is carried outat step S9. The scan required for the Z-map must measure the height ofsufficient points to enable the wafer to be positioned during exposureat the desired accuracy. It is also important that the points measuredcover the actual area where the wafer is to be exposed; measurementstaken over non-exposure areas, such as scribe lanes and so-called mousebites, may give misleading results. Accordingly, the height mapping scanmust be optimized to the specific pattern of exposure areas on the waferat hand; this is described further below.

Once the Z-map is completed, the advance alignment measurements, stepS10, are carried out before the substrate table is swapped, at step S11,to the exposure position. In the advance alignment process, thepositions of a number of alignment markers on the wafer relative to thereference markers F located on the TIS fiducial (physical referencesurface) fixed to the substrate table are accurately determined. Thisprocess is not particularly relevant to the present invention and so isnot described further herein.

In the swap procedure, the substrate table carrying the height-mappedwafer arrives at the exposure station, step S13 in FIG. 10. A coarseposition determination of the substrate table is made at step S14 and,if desired or necessary, a new mask MA is loaded onto the mask table MT,step S15. The mask loading process may be carried out, or at leastbegun, simultaneously with the substrate table swap. Once a mask is inposition and a coarse position determination, step S14, has been made, afirst TIS scan is carried out using sensor TIS1 at step S16. The TISscan measures the vertical and horizontal position of the substratetable at which the TIS is located in the aerial image focus of theprojection lens, as described above, yielding a focal plane reference.Since the height map generated as step S9 in FIG. 9 is referenced to thephysical surface in which the TIS is located, the vertical positions ofthe substrate table necessary to put the wafer surface in the focalplane for the different exposure areas are directly derived. A secondTIS scan, step S17, is also carried out using sensor TIS2, yielding asecond point for referencing a focal plane.

Once the TIS scans have been completed and the focal plane determined,the exposure process S18 is carried out, optionally after any desired ornecessary system calibrations in step S19 (e.g. adjustments to correctfor lens heating effects). The exposure process will generally involvethe exposure of multiple exposure areas using one or more masks. Wheremultiple masks are used, after mask exchange S20, one TIS scan S17 canbe repeated to update any focal plane changes. Between some or allexposures, the system calibration step S19 may also be repeated. Aftercompletion of all exposures, the substrate table carrying the exposedwafer is swapped at step S13 for the substrate table carrying the waferthat has meanwhile been undergoing steps S1 to S10 of FIG. 9. Thesubstrate table carrying the exposed wafer is moved to the loadingstation and the exposed wafer taken out so that a fresh wafer can beloaded and the cycle can resume.

To explain the wafer height mapping scan of step S9 of FIG. 9, FIG. 11shows an example of a pattern of exposure areas C of various shapes andsizes arranged on a wafer to make best use of the silicon area. Thedifferent exposure areas C are separated by scribe lanes SL andgenerally-triangular unused areas, known as “mouse-bites” are inevitablyleft between the rectangular exposure areas and the curved edge of thewafer. The scribe lanes are where the wafer will be cut once allproduction processes have been completed (so as to separate thedifferent devices) and some cutting techniques may require that thescribe lanes in one direction all span the entire width of the wafer; inthat case it is convenient to orient these full wafer-width scribe lanesparallel to the scanning direction (e.g. the Y direction) if theapparatus is to be used in step-and-scan mode. The scribe lanes andmouse bites may not be exposed, and so after the wafer has beensubjected to a few process steps or depositions of layers they may havevery different heights and surface properties than the exposure areas C.Accordingly it is important to disregard any height measurements inthese areas, which are not going to be exposed.

A presently preferred embodiment of the level sensor uses a linear arrayof, e.g., nine optical spots arranged perpendicular to the scanningdirection to measure the height at nine points (areas) simultaneously.(Note that the Z-interferometer data can also be interpolated to providecorresponding Z-position data of the substrate table at an array ofcorresponding level sensor points.) The array of spots is of a sizesufficient to cover the width of the widest exposure area that can beexposed in the apparatus.

The presently preferred scanning scheme is to scan the array of spots ina meander path 50 such that the center spot of the array passes alongthe midline of each column of exposure areas; this midline correspondsto the midline of the illuminated slit in the exposure process. The datathus generated can be directly related to the exposure scan with aminimum of rearrangement or calculation. This method also eliminatespart of the mirror unflatness effect, since, at both measurement andexposure stations, scans are carried out with the Z-interferometer beampointing at the same position on the mirrors 31 attached to thesubstrate table. If the column of dies is narrower than the array ofspots of the level sensor, data obtained from the spots not lying whollywithin the exposure area are ignored. In other embodiments of the levelsensor it may be possible to adjust the width of the array of spots tomatch the width of the exposure areas.

If a wafer has some exposure areas whose center lines are offset in thedirection perpendicular to the scanning direction from those of theremainder, a modified scanning scheme may be used to advantage. Thissituation is illustrated in FIG. 12 which shows one row of dies E whosecenter lines are offset from the remaining dies D. In such a case, themap can be created more quickly and with fewer accelerations for thesubstrate table by scanning two meander paths. One path, referenced 52in FIG. 12, covers one set of exposure areas D and the other, referenced53, covers the others E. Of course, other arrangements of the exposureareas may require further modifications to the scanning scheme.

Where the level sensor has a limited linear or linearized range, whichis likely the case, the substrate table WT must be scanned underneath itat a vertical position that brings the wafer surface into that range.Once the wafer surface has been found it is a simple matter, by means ofa closed feedback loop of the level sensor reading to the substratetable positioning system, to adjust the vertical position of thesubstrate table WT to keep the wafer surface in the linear or linearizedrange but it is not so simple to find the wafer surface when the levelsensor first moves onto an exposure area from outside the wafer. In ameander path there are several such in-points, referenced 51 andindicated by arrows on the meander path 50 in FIG. 11, compounding theproblem.

To find the wafer surface at the in-points 51 it is possible to providea capture spot in advance of the main level sensor spot array. Thereflection of the capture spot on the wafer is then directed to adetector that has a wider capture range than is the case for the mainspots. This, however, requires additional hardware: a capture spot onboth sides of the main spots (before/after) or a restriction to scanningin only one direction. An alternative, not necessarily requiringadditional hardware, is to stop the substrate table close to eachin-point, perform a wafer capture and measure the wafer surface in thelinear or linearized range of the level sensor to approximate the wafersurface position at the in-point. This however slows down themeasurement procedure significantly, which may have undesirableconsequences for throughput.

In this embodiment of the invention, these problems are avoided byperforming a global level contour scan mentioned above (step S5 in FIG.9) after the wafer surface is captured. The global level contour scan isexplained further with reference to FIG. 13.

For the global level contour scan the substrate table is firstpositioned so that a convenient point (preferably near the edge) withinan exposure area C is underneath a single capture spot and the mainspots of the level sensor (spot array). The wafer surface is found, e.g.by scanning the substrate table vertically until the wafer surface iscaptured and comes within the linear or linearized range of the mainspots, and then the substrate table is scanned so that the central spot41 traverses a path 60 around the inside of the perimeter of the totalexposure area. The capture procedure is described further below.Measurements of the wafer surface height are taken at defined positionsaround the scan. Where other spots of the array as well as the centerspot fall over (exposure areas of) the wafer, the measurements fromthese spots, as well as the central one, can also be taken. However,measurements should not be taken from spots falling outside the exposureareas. As illustrated, the global level contour path 60 is a windingpath following the edges of the exposure areas quite closely; however asmoother path may also be employed and, particularly when the wafer iswell filled with exposure areas, a circular course 61 may well sufficeand be more convenient. The global level contour may also be arranged asa circle passing over mouse bites, in which case measurements are nottaken over the mouse bites, or the data of any measurements taken onmouse bites are disregarded in calculation of the global height and tiltof the wafer.

The data gathered in the global level contour scan are used for twopurposes. Firstly data relating to the wafer height in the vicinity ofthe in-points 51 (see FIG. 11) of the height mapping scan to be carriedout later are used to predict the wafer height at the in-points 51 sothat the substrate table can be brought to the correct height to get thewafer surface position in the linear or linearized level sensor rangeduring the mapping scan. In most cases only a few data points arerequired for this and they need not be particularly close to thein-points to allow a sufficiently accurate prediction of the waferheight to be determined by interpolation or extrapolation. It is alsodesirable to know the local Ry tilt at the in-points 51 for the heightmapping scan, since the level sensor has an array of spots in theX-direction which (preferably) all need to be brought within theirlinear or linearized ranges. If the global level contour scan isparallel, or nearly parallel, to the Y direction in the vicinity of anyin-point, the Ry tilt cannot be accurately determined using dataobtained from only a single spot. Where a level sensor having an arrayof measurement spots spaced apart in the X direction, such as thatdescribed below, is used, data from multiple spots can be used todetermine a local Ry tilt. Of course, data from spots lying within theexposure area are selected if part of the array falls outside that area.

The second use of the global level contour data is to determine aglobal, or average, height and tilt (around 2 axes) for the whole wafer.This is done by known mathematical techniques, e.g. a least-squaresmethod, to determine a plane that most closely fits the wafer heightdata gathered. If the global tilt (sometimes referred to as the “wedge”)is greater than a predetermined amount, this may well indicate anincorrect loading procedure. In that case the wafer can be unloaded andreloaded for a retry and even rejected if it continues to fail. Theglobal height and tilt information is used to focus an advance alignmentsensor used in step S10 of FIG. 9 to accurately determine the spatialrelationship of alignment markers on the wafer to reference markers onthe substrate stage. The advance alignment sensor and process aredescribed in greater detail in WO 98/39689 (P-0070).

During a wafermap scan, the level sensor 10 provides continuous Z and Ryfeedback signals to the substrate table to keep the level sensor 10 inits linear or linearized range. If this feedback loop stops (the levelsensor 10 doesn't supply correct numbers) the table is controlled byfollowing a path corresponding to the global wafer wedge (a Z profileaccording to global Rx).

A presently preferred embodiment of the level sensor 10 is illustratedin FIG. 14 and will be explained below additionally with reference toFIGS. 14A to 14G, which show aspects of the operation of the sensor.

Level sensor 10 comprises a beam generation branch 11 which directs ameasurement beam b_(LS) onto the wafer W (or the physical referenceplane when the vertical position of that is being measured, or anyreflecting surface) and a detection branch 12 which measures theposition of the reflected beam, which is dependent on the verticalposition of the wafer surface.

In the beam generation branch, the measurement beam is generated bylight source 111, which may be an array of light emitting or laserdiodes, or generated elsewhere and passed to “illuminator” 111 byoptical fibers. The beam emitted by light source 111 preferably containsa wide band of wavelengths, e.g. from about 600 to 1050 nm, so as toaverage out any wavelength dependence of interference effects from thewafer surface, particularly after some process steps have beencompleted. Illumination optics 112, which may include any suitablecombination of lenses and mirrors, collect the light emitted by lightsource 111 and evenly illuminate projection grating 113. Projectiongrating 113 is shown in greater detail in FIG. 14A and consists of anelongate grating 113 a, which may be divided to generate an array ofseparate/discrete spots, with grating lines parallel to its axis, and anadditional aperture 113 b which forms a capture spot ahead of the maindetection spot array on the wafer. The period of the grating will bedetermined in part by the accuracy at which the wafer surface positionis to be measured and may, for example be about 30 microns. Theprojection grating is positioned with a small rotation around itsoptical axis such that the grating lines projected on the wafer are notparallel to any wafer coordinate axis, thereby to avoid interferencewith structures on the wafer which are along the x or y direction.Projection lens 114 is a telecentric system which projects an image ofthe projection grating 113 onto the wafer W. Projection lens 114preferably consists essentially or only of reflecting optical elementsso as to minimize or avoid chromatic aberration in the projected image;since the projection beam is broadband these cannot easily be eliminatedor compensated for in a refractive optical system. Folding mirrors 115,116 are used to bring the projection beam b_(LS) into and out of theprojection lens 114 and permit a convenient arrangement of thecomponents of the beam generation branch.

The projection beam b_(LS) is incident on the wafer at a fairly largeangle, α, to the normal, e.g. in the range of from 60° to 80°, and isreflected into the detection branch 12. As shown in FIG. 14B, if thewafer surface WS shifts in position by a distance Δh to position WS′,then the reflected beam r′ will be shifted relative to the beam r, priorto the shift in the wafer surface, by a distance 2.Δh. sin (α). FIG. 14Balso shows the appearance of the image on the wafer surface; because ofthe large angle of incidence, the image is spread out perpendicular tothe grating lines.

The reflected beam is collected by detection optics 121 and focused ondetection grating 126, which is essentially a copy of projection grating113 and is sub-divided to correspond to the spot-array pattern.Detection optics 121 are directly complementary to projection optics 114and will also consist essentially or only of reflective elements, tominimize chromatic aberration. Again folding mirrors 122, 123 may beused to enable a convenient arrangement of the components. Betweendetection optics 121 and the detection grating 126 are positioned alinear polarizer 124 to polarize the light at 45° and a birefringentcrystal 125 which causes a shear perpendicular to the grating linesequal in magnitude to the grating period between the horizontal andvertical polarized components of the light. FIG. 14C shows the beam asit would be at the detection grating 126 without the birefringentcrystal; it is a series of alternating light and dark bands with thelight bands polarized at 45°. The birefringent crystal 125 shifts thehorizontal and vertical polarization states so that the light bands ofthe horizontal polarization component fill the dark bands of thevertical polarization component. As shown in FIG. 14D, the illuminationat the detection grating 126 is therefore uniform grey but has stripesof alternating polarization state. FIG. 14E shows the detection grating126 overlaid on this pattern, which depends on the vertical position ofthe wafer surface; when the wafer is at a nominal zero verticalposition, the detection grating 126 will overly and block half of thelight bands of one polarization state, e.g. the vertical, and half ofthe other state.

The light passed by the detection grating 126 is collected by modulationoptics 127 and focused on detector 128. Modulation optics include anpolarization modulation device driven by an alternating signal, e.g.with a frequency of about 50 kHz, so as to pass the two polarizationstates alternately. The image seen by the detector 128 thereforealternates between the two states shown in FIG. 14F. Detector 128 isdivided into a number of regions corresponding to the array of spotswhose height is to be measured. The output of a region of detector 128is shown in FIG. 14G. It is an alternating signal with period equal tothat of the modulating optics and the amplitude of the oscillationsindicates the degree of alignment of the reflected image of theprojection grating on the detection grating, and hence the verticalposition of the wafer surface. As mentioned above, if the wafer surfaceis at the nominal zero position, the detection grating 126 will blockout half of the vertical polarization state and half of the horizontalpolarization state so that the measured intensities are equal and theamplitude of the oscillating signals output by the detector regions willbe zero. As the vertical position of the wafer surface moves away fromthe zero position, the detection grating 126 will begin to pass more ofthe horizontally polarized bands and block more of the verticallypolarized bands. The amplitude of the oscillations will then increase.The amplitude of the oscillations, which is a measure of the verticalposition of the wafer surface, is not directly linearly related to thevertical position of the wafer surface in nanometers. However, acorrection table or formula can readily be determined on initial setupof the apparatus (and periodically recalibrated, if desired ornecessary) by measuring the constant height of the surface of a baresilicon wafer at various different vertical positions of the substratetable, using the calibrated Z-interferometer and uncalibrated levelsensor 10.

To ensure that the measurements of the level sensor and theZ-interferometer are taken simultaneously, a synchronization bus isprovided. The synchronization bus carries clock signals of a very stablefrequency generated by a master clock of the apparatus. Both the levelsensor and Z-interferometer are connected to the synchronization bus anduse the clock signals from the bus to determine sampling points of theirdetectors.

The capture spot 113 b passed by the projection grating 113 passes thedetection grating, where it is incident on three separate detectionregions, two 131, 133 set high and one 132 set low, as shown in FIG.15A. The output from the low detection region is subtracted from thoseof the high regions. The capture spot detector regions are arranged sothat when the wafer surface is at the zero position, the capture spotfalls equally on the high and low detection regions and the subtractedoutput is zero. Away from the zero position, more of the capture spotwill fall on one of the detection regions than the other and thesubtracted output will increase in magnitude with its sign indicatingwhether the wafer is too high or too low. The dependence of thesubtracted detector output d_(cap) on substrate table position Z_(IF) isillustrated in FIG. 15. This form of detector output allows a fasterzero capture method than a conventional servo feedback. According to theimproved method, referred to as “move-until”, when the capture spotdetector indicates that the wafer surface is too high or too low, theZ-position actuators of the substrate table are instructed to move thestage in the appropriate direction to bring the wafer surface into thelinear or linearized range of the main level sensor array. The movementof the wafer stage continues until the output of the capture spotdetector d_(cap) passes a trigger level t_(h) or t_(l) according towhich direction it is traveling. Crossing the trigger level causes theapparatus control to issue a command to the Z-position actuators tobegin a braking procedure. The trigger levels are set so that, in theresponse time and the time taken to brake the stage motion, the stagewill move to, or close to, the zero position. Thereafter the stage canbe brought to the zero position under control of the more accurate mainlevel sensor spots. The trigger points will be determined in accordancewith the dynamics of the stage and need not be symmetrically spacedabout zero detector output. This “move-until” control strategy enables arapid and robust zero capture without requiring a linear measurementsystem, and can be used in other situations.

The level sensor described above can be further optimized to improve itsperformance. Improvement in accuracy in the scan (Y) direction can beeffected by appropriate signal filtering and this may be adapted tospecific process layers observed on partly processed wafers. Additionalimprovements (for specific process layers) in all directions may beobtained by optimization of the measurement spot geometry, which can beadjusted by changing the illumination optics 112 (to adjust theuniformity and/or angular distribution of the illumination light on theprojection grating 113), by changing the projection grating 113 or byadjusting the detection system (size, position and/or angular resolutionof the detector and the number of detectors).

A presently preferred form of the confidence sensors 20 a, 20 b isillustrated in FIGS. 16 and 17. The beam generation branch 21 comprisesa light source 211 (e.g. a solid state laser diode or super-luminescentdiode) which emits light of limited bandwidth. It is convenientlysituated away from the metrology frame and its output brought to thedesired point by an optical fiber 212. The light is output from fiberterminator 213 and directed onto a beam splitter 215 by collimatingoptics 214. Beam splitter 215 creates two parallel measurement beamsb_(cs1) and b_(cs2) which are focused to evenly illuminate respectivespots 23 on the wafer W by telecentric projection optics 216. Since themeasurement beams of the confidence sensor have a limited bandwidth,projection optics 216 can conveniently employ refractive elements.Detection optics 221 collect the reflected beams and focus them at theedge of detection prism 222 which is positioned between detectors 223,224 and detection optics 221. As shown in FIG. 17, which is a side viewof detection prism 222 and detector 223, a measurement beam is incidenton the back of detection prism 222 and exits through angled faces 222 a,222 b. Detector 223 consists of two detector elements 223 a, 223 bpositioned so that light emerging from face 222 a of detection prism 222reaches detector element 223 a and that emerging from face 222 b reachesdetector element 223 b. Detector 224 is similar. Outputs of detectorelements 223 a and 223 b are intensity-scaled and subtracted. When thewafer surface is at the zero position, the measurement beam fallssymmetrically on faces 222 a, 222 b of detection prism 222 and equalamounts of light will be directed to detector elements 223 a and 223 b.These will then give equal outputs and so the subtracted output will bezero. As the wafer surface moves away from the zero position, theposition of the reflected beam will move up or down and fall more on oneof faces 222 a, 222 b than on the other resulting in more light beingdirected to the respective detector element so that the subtractedoutput will change proportionally. A tilt of the wafer can be determinedby comparison of the outputs of detectors 223 and 224.

This arrangement provides a simple and robust height and level detectorthat can be used as the confidence sensor in the second embodiment ofthe present invention as well as in other applications. The confidencesensor is primarily intended for initial set up and periodic, e.g.monthly, recalibration of the Z-interferometers of the measurement andexposure stations. However, the confidence sensor described above has awider capture zone and more rapid response than the TIS used for precisedetermination of the position of the focal plane of the projection lensPL relative to substrate table WT. Accordingly, the confidence sensor 20a can advantageously be used, when the substrate table is first swappedto the exposure station, to make a coarse determination of the verticalposition of the TIS. The height measured by the confidence sensor isrelated to previously measured best focus position(s) and used topredict a starting point and range for the TIS scan near the expectedposition of the best focal plane. This means that the TIS scan,described above, can be made shorter and hence quicker, improvingthroughput.

A beam splitter 215 that can be used in the confidence sensors is shownin FIG. 18. A beam splitter is composed of a number of prisms from thesame glass and preferably of equal thickness. The basic operationprinciple is described using a beam splitter consisting of 3 prisms 51,52, 53. Prism 51 is trapezoidal in cross-section and the input beam 54is incident normally on its top face 55 near one side. The position ofinput beam 54 is such that it meets one side face 56 of first prism 51which is at 45° to the top face 55. Second prism 52 is joined onto sideface 56 of first prism 51 and the join is coated so that a desiredproportion of the input beam (half in the present embodiment) continuesinto second prism 52 to form beam 57 whilst the remainder is reflectedhorizontally within first prism 51 to form beam 58. Beam 58 reflected infirst prism 51 meets the second side face 59 of that prism, which isparallel to the first side face 56 and is reflected downwards out of thelower face of first prism 5l and through top and bottom faces of thirdprism 53 which are parallel to top face 55 of first prism 51. Secondside face 59 may be coated as desired or necessary to ensure totalinternal reflection of beam 58. Beam 57, which passed into second prism52, is reflected internally by two parallel faces of second prism 52,which are perpendicular to side face 56 of first prism 51, and emergesfrom the bottom face of second prism 52 which is parallel to the topface 55 of first prism 51. Beams 57 and 58 are thereby output inparallel, but displaced. The separation between beams 57, 58 isdetermined by the sizes of prisms 51 and 52. Prism 53 is provided toequalize the optical path lengths of beams 57, 58 so that the imagingoptics for both beams can be identical. Prism 53 also supports prism 52as illustrated but this may not be necessary in some applications. Toenhance the reflection of beam 57 at the surface where prisms 52 and 53meet, a void may be left or a suitable coating provided.

Beam splitter 50 is simple, robust and easy to construct. It providesoutput beams in parallel (whereas a conventional cubic beam splitterprovides perpendicular beams) and with equal path length. The splittingsurface can be made polarization selective or not, and in the lattercase can divide the input beam intensity evenly or unevenly as desired.

It is a feature of the level and confidence sensors described above, aswell as other optical height sensors, that they are insensitive to tiltof the wafer stage about an axis perpendicular to the Z-directiondefined by the intersection of the wafer surface WS and the focus planeof the measurement spot of the level sensor 10. This is due to the factthat the sensors measure a height over the area of the measurement spotextrapolated to the spot's focus axis. The tilt insensitivity can beused to calibrate the Z-interferometers and the optical sensors towardseach other in the XY plane. The procedure for such calibration isdescribed with reference to FIG. 19 and the level sensor, but a similarprocedure can be used with the confidence sensor or any other similaroptical sensor.

The positioning system of the substrate table is linked to themulti-axis interferometer system of which the Z-interferometer is apart, and can be set to apply a rotation about a selected axis in the XYplane using spaced-apart Z-actuators. To align the Z-interferometermeasurement position with the level sensor measurement spot, thepositioning system is used to rotate the stage about an axis passingthrough the Z-interferometer measurement position and parallel to, forexample, the Y axis. The Z position of the table as measured by theZ-interferometer will remain unchanged during this tilt. If the levelsensor and Z-interferometer are exactly aligned, then the wafer surfaceposition will also remain unchanged. However, if the level sensormeasurement position is offset from the Z-interferometer position by anamount δX, as shown in FIG. 19, then tilting the substrate table WT tothe position shown in phantom in that Figure may cause a change δW_(LS)in the level sensor output. The offset δX, and the offset δY in the Ydirection, can therefore be quickly determined by detecting any changein level sensor output with tilts about two, preferably perpendicular,axes passing through the Z-interferometer position. The parameters ofthe interferometer system or the level sensor 10 can then be adjusted toensure that the Z-interferometer measurement position is exactlyopposite the level sensor measurement position.

Where the level sensor uses an array of measurement spots, it cannotalways be ensured that the spots are exactly aligned. The abovetechnique can therefore be used to determine any offsets of theindividual spots from the nominal position with respect to theZ-interferometer position. This information can then be used to correctthe height map or the level sensor output.

It is noted that properties of a dioptrical element may be related to aninteraction of light which refracts at the interface of the element andthe ambient. In an interpretation of classical physics, Snell's lawdescribes that at an interface between two different media, thepropagation direction of light rays traversing from one medium to theother changes from the one medium to the other, due to a difference ofrefractive index between the two media.

Thus, when a lens element is used in air ambient, the optical propertiesof this lens may be different from those in pure nitrogen, helium, orargon. Also, the working ambient may comprise a liquid to improve theoptical performance (such technology is known as immersion lithography).Moreover, other gases or liquids may be considered for differentpurposes (for example in relation to specific properties such as thermalconductance or electrical isolation, etc). Similarly, when the lenselement is placed in vacuum the refraction of light in the lens may alsobe different.

As mentioned above, if a lithographic apparatus is to be used duringdevice manufacturing in a specific ambient such as nitrogen, differentfrom the ambient in which the lithographic apparatus is tested andcalibrated, it may be desirable to make a correction during calibrationto compensate for deviations while working in the manufacturing ambient.Such a correction may consist of a mechanical adaptation of a positionof the optical system relative to the projection plane for the image orthe addition of an extra optical component.

As an illustration, FIG. 22 depicts a dioptrical element. As adioptrical element a convex lens CL is shown. An object Obj is imaged bylens CL as an image Im when placed in air ambient. However, when placedfor example in vacuum, the refraction of light by CL changes slightly,and the distance of the image in vacuum Im2 to the lens CL will besmaller than the distance between Im and CL in air ambient (therefractive index of air is approximately 1.0002926, while the refractiveindex of vacuum is 1.0).

Although this effect may be small (not shown on scale in FIG. 22), incircumstances as exist in a lithographic apparatus with a very shallowdepth of focus and for small features on the order of e.g. one hundrednanometers, the change of image quality (from test ambient to workingambient) may be significant when no correction is taken into accountduring a calibration (in test ambient).

FIG. 23 depicts an example of a catoptrical system in accordance with anembodiment of the present invention. The catoptrical system RE for lensCL comprises only reflective elements and provides an imaging functionsimilar to that of lens CL and simultaneously solves the aforementionedcorrection problem.

Catoptrical system RE is shown in a cross-sectional plane X′Y′ andcomprises a first planar mirror Mp1, and second planar mirror Mp2, afirst concave mirror Mh1, and a first convex mirror Mb1. In FIG. 14, theembodiment of catoptrical system RE is shown as projection optics 114and detection optics 121.

On an optical axis OA parallel to a first direction X′, the first planarmirror MP1 is located at a first position X1 under a first angle A1. Inthe plane X′Y′ the first concave mirror Mh1 having a first radius Rh islocated at a second position X2. The position X2 of concave mirror Mh1and the first angle A1 of first planar mirror Mp1 are chosen in such away that a light beam parallel to the optical axis OA after reflectionon first planar mirror Mp1 will be directed towards concave mirror Mh1.

After reflection of the beam of light LB on concave mirror Mh1, the beamof light LB is directed to first convex mirror Mb1 (due to the firstradius Rh of Mh1). First convex mirror Mb1 comprises a second radius Rband is positioned at a third position X3.

After reflection of the light beam LB on convex mirror Mb1, the lightbeam is directed to second planar mirror Mp2 (due to the second radiusRb of Mb1) via first concave mirror Mh1. Second planar mirror Mp2 ispositioned at a fourth position X4 under a second angle A2. In thesituation shown, A2=180°−A1, mirrored relative to first planar mirrorMp1, using the Y′-direction as mirror plane.

After reflection at second planar mirror Mp2, the beam of light LB isagain substantially parallel to the optical axis OA. The object Obj isimaged by catoptrical system RE as image Im in a similar way as byconvex lens CL.

Because each sub-element Mp1, Mp2, Mh1, Mb1 of catoptrical system REitself is a reflective element, the influence of the ambient'srefractive index on the image position can be minimized or evenneutralized. For each mirror element, refraction is governed by a simplemirror operation: “angle of incidence is angle of reflection,”independent of the refraction index of the ambient where such a mirrorelement is located. In an arrangement according to an embodiment of thepresent invention, the sub-elements Mp1, Mp2, Mh1, Mb1 each comprise areflecting layer on their respective front side, which is directedtowards the incoming light beam.

The use of a catoptrical system (as shown here, for example) as imagingelement which may operate in a working ambient other than the test andcalibration ambient provides a solution for the application of acorrection of the imaging element for the working ambient. A correctionmay not be necessary anymore. Only a calibration for correct imaging inthe test and calibration ambient may be needed.

The catoptrical system RE shown here functions in a similar way as aconvex lens. As will be appreciated by persons skilled in the art, thepresent invention is not limited to this embodiment of the catoptricalsystem RE. The catoptrical system RE may be arranged in such a way thata function similar to other dioptrical elements, such a concave,planoconvex, planoconcave or other type of lens or lens system(comprising a plurality of lenses) is achieved.

Thus, a complete optical system for optical detection or optical sensingthat is equipped with dioptrical elements may be replaced by a furthercatoptrical system RE in accordance with the present invention, whereineach dioptrical element is replaced by one or more reflective elementswhich provide an identical imaging function as that dioptrical element.In FIG. 14, the complete optical system can be replaced by a furtherembodiment of a catoptrical system which comprises the embodiment of thecatoptrical system RE as described above, and additional reflectiveoptical elements for guiding a light beam through the system without theneed for substantially any corrections relating to the working andtesting ambients.

Also, in dependence on the actual optical path for the light beam, thecatoptrical system RE may comprise a different number of sub-elements.As will be appreciated by persons skilled in the art, the catoptricalsystem RE comprises at least one of the concave mirror Mh1 and theconvex mirror Mb1 to replace one of the lens types as mentioned above.

Moreover, the catoptrical system RE may also be arranged to functionlike other optical elements which display a refraction at one or moreinterfaces between the medium of the element and a surrounding ambient,for example, like a plane-parallel plate element.

Components such as a Fresnel lens or other complex optical componentscan be replaced by catoptrical elements as well.

By such a catoptrical system RE as described above, any opticaldetection or sensing system in a lithographic apparatus where some kindof imaging is involved such as an alignment detector, an overlay sensor,or a leveling sensor, may be adapted. Then, calibration of an opticaldetection or sensing system equipped with a suitable catoptrical systemRE may simply be carried out in air ambient, and without any furtheraction may be used in working ambient during the manufacturing cycle ascarried out by the lithographic apparatus.

It is noted that embodiments of the present invention may be applicablein other fields of technology (where a transition to another ambient,liquid or vacuum would influence the refraction conditions of dioptricalelements), such as in optical analysis and detection systems placed invacuum.

Moreover, it is noted that in a lithographic apparatus according to anembodiment, the catoptrical system RE may be placed within the apparatuson a deformation-reducing sub-frame which may prevent any deformationsof the catoptrical system RE as inferred by a change of ambient (orchange to vacuum conditions).

A third embodiment employs the levelling principle of the firstembodiment and is the same as that embodiment except as described below.The third embodiment may also make use of the hardware and refinementsof the second embodiment, described above. However, the third embodimentmakes use of an improved method for optimization of the exposure path.This is explained below with reference to FIG. 20.

As discussed above, it is convenient and valid to consider that thesubstrate stage is stationary and that the exposure slit image moves,even though in practice it is the wafer that moves. The explanationbelow is given from this view point.

FIG. 20 illustrates the notations used below. It should be noted that,although the slit image SI is depicted for clarity in FIG. 20 spacedfrom the wafer surface, the aim of the optimization procedure is toensure that during an exposure the focus plane of the slit imagecoincides as far as possible to the wafer surface. Considering a onedimensional wafer whose surface is defined by w(y) and a slit image SI,the moving average (over time) defocus MA(y) corresponding to acoordinate on the wafer can be calculated from:

$\begin{matrix}{{M\;{A(y)}} = {\frac{1}{s}{\int_{{- s}/2}^{s/2}{\left\lbrack {{w(y)} - \left\lbrack {{z\left( {y + v} \right)} - {{v \cdot R}\;{x\left( {y + v} \right)}}} \right\rbrack} \right\rbrack{\mathbb{d}v}}}}} & (2)\end{matrix}$

where the integral is taken over the slit size, s, in the scan directionand the integrand w(y)−[z(y+v)−v.Rx(y+v)] is the focus error on a pointof the wafer at a certain moment in time. Similarly, the moving standarddeviation for a point on the wafer can be defined as:

$\begin{matrix}{{{MSD}^{2}(y)} = {\frac{1}{s}{\int_{{- s}/2}^{s/2}{\left\lbrack {{w(y)} - \left\lbrack {{z\left( {y + v} \right)} - {{v \cdot R}\;{x\left( {y + v} \right)}}} \right\rbrack - {{MA}(y)}} \right\rbrack^{2}{\mathbb{d}v}}}}} & (3)\end{matrix}$

which is the defocus variation in time during the actual exposure ofthat point on the wafer. To minimize the difference between the plane ofthe exposure slit image and the wafer, a quadratic defocus term is used,defined as follows:

$\begin{matrix}{{M\;{F^{2}(y)}} = {\frac{1}{s}{\int_{{- s}/2}^{s/2}{\left( {{w(y)} - \left\lbrack {{z\left( {y + v} \right)} - {{v \cdot R}\;{x\left( {y + v} \right)}}} \right\rbrack} \right)^{2}{\mathbb{d}v}}}}} & (4)\end{matrix}$

where MF(y) is called the moving focus. It will be seen that MF(y) canalso be written in terms of MA(y) and MSD(y) as follows:MF ²(y)=MA ²(y)+MSD ²(y)  (5)

This means that in the optimization of the exposure path andminimisation of the moving focus over the exposure area, both the movingaverage and the moving standard deviation are taken into account, incontrast to the simpler least-squares optimization of the firstembodiment, which neglects any time, and thus scanning, integration.Equations [3] and [4] can easily be extended to two dimensions by addingRy(t) dependency and integrating MF over X from −W/2 to +W/2, where W isthe width of the slit in the X-direction. To calculate the optimizationit is convenient to use a frequency domain representation. Calculationin the frequency domain also enables high-frequency variations in thesetpoints, that would result in excessive substrate stage accelerationsin any or all of the degrees of freedom, to be filtered out, such thatthe exposure path is optimized for the performance of the substratetable positioning system.

In the above discussion, the optimum focus of the exposure slit image isassumed to conform to a plane; however, this is not necessarily thecase: the optimum focus may in fact lie on an arbitrary surface,resulting in a so-called focal plane deviation (FPD). If the contour ofthat surface over the exposure slit area can be measured using the TISto create a focus map f(x,y), or calculated, then the resulting data orequations can be added to the equations above so that the wafer motionis optimized for the actual optimum focal surface.

The optimization technique of the third embodiment can result in betterfocus for scanning systems and smoother substrate stage trajectories,increasing throughput and yield.

In a fourth embodiment, the level sensor is provided with additionalfeatures to counteract errors in the measurement of the wafer surfaceposition that may be caused by interference between the beam reflectedby the top surface of the resist layer and the beam refracted into theresist layer and reflected by its bottom surface. Otherwise, the fourthembodiment may be the same as any of the first to third embodimentsdescribed above.

The interference of beams reflected from said top and bottom surface islargely dependent on the resist properties and wafer surface properties,as well as on the optical wavelength and angle of incidence of themeasurement beam. Broadband light sources and detectors are currentlyused to average out such single-wavelength interference effects.Improvement of this averaging principle can be realized if the wafersurface position is measured in a spectrally resolved manner, whereby adistinct measurement is performed for a number of wavelengths in thebroadband measurement beam. To achieve this, it may be desired ornecessary to make a temporally or spatially separated wavelength (color)system for measuring the wafer surface position. This may necessitatechanges such as the following to the level sensor's measurementprinciple.

A first possible change to the level sensor is to replace the continuousbroadband light source by one capable of selectively generating lightbeams of different wavelength ranges (colors). This can, for example, beachieved by selectively interposing different color filters (e.g. on acarousel) at a suitable point in the level sensor's illumination system,by the use of several independently selectable light sources, by using awavelength-tunable light source, or by using a selected beam portionfrom a rotating/vibrating prism located in a small broadband beam. Thelevel sensor is then used to take several measurements of the wafersurface at each point, using different wavelengths of light in themeasurement beam.

Another option is to replace the broadband detector by one capable ofselectively detecting light of different wavelength ranges (colors).This can be achieved, for example, by placement of color filters in thedetection optics before the detector, by spatially splitting themeasurement beam for different wavelengths using a prism and thendetecting the different-wavelength beams on separate detectors, or byany other way of spectrally analyzing the broadband-reflected beam tomeasure the wafer surface position.

Naturally, it is also possible to use a combined approach, whereby boththe projection system and the detection system are adapted to achievespectral resolution.

In the absence of interference effects, each measurement (for eachwavelength) should give the same result; consequently, if differentresults are obtained in such measurements, this indicates the presenceof effects as referred to in the first paragraph above. An improvedwafer surface position measurement can then be derived using a varietyof techniques. For example, discrepant results may be corrected ordiscarded. Majority voting techniques may also be used. Alternatively,on the basis of a spectral measurement of the wafer surface position,one might even derive real positions by means of a model describing thespectral response of the resist and the wafer surface properties.

Since the described interference effect also depends on the angle ofincidence of the measurement beam on the wafer surface, one might alsowant to vary this angle of incidence so as to evaluate the effect andthen correct it. Accordingly, a further possible change to the levelsensor is to adapt it such that the wafer surface position can be madeusing measurement beams at different angles of incidence. One way toachieve this is to define multiple measurement beams having differentangles of incidence for the same spot on the wafer, but separateprojection and detection optics systems. Alternatively, one can changethe optical system so that the same projection and detection systemsencompass the different optical axes pertaining to the variousmeasurement beams. Another option, which generates temporally varyingangles of incidence, is to use rotating/translating folding mirrors (orother movable components) in the optical systems of the level sensor.

As with the wavelength dependence described above, in the absence ofinterference effects, measurements at different angles of incidenceshould give the same result. Therefore, any discrepancies (variationwith angle of incidence) can be avoided, compensated for, or modeled inthe same way.

The above-mentioned additional features and improvements may, of course,be used together or separately, and in other optical sensors than thosedescribed here.

A fifth embodiment of the invention is shown in FIG. 21. The fifthembodiment of the invention is a lithography apparatus employing, as theexposure radiation, extreme ultraviolet (EUV) radiation, e.g. ofwavelength in the range of 9 to 16 nm, and a reflective mask MA′.Functionally at least, the components of the fifth embodiment aregenerally the same as those of the first embodiment but they are adaptedto the exposure radiation wavelength used and their arrangement isadjusted to accommodate the beam path necessitated by the use of areflective mask. Particular adaptions that may be desired or necessarymay include optimizing the illumination and projection optics IL′, PL′to the wavelength of the exposure radiation; this will generally involvethe use of reflective rather than refractive optical elements. Anexample of an illumination optical system IL′ for use with EUV radiationis described in European Patent Application 00300784.6.

An important difference between lithography apparatus using reflectivemasks and those using transmissive masks, is that with the reflectivemask, unflatness of the mask results in position errors on the waferthat are multiplied by the optical path length of the downstream opticalsystem, i.e. the projection lens PL′. This is because height and/or tiltdeviations of the mask locally change the effective angle of incidenceof the illumination beam on the mask and hence change the XY position ofthe image features on the wafer.

According to the fifth embodiment of the invention, the effects ofunflatness of the mask are avoided or alleviated by making a height mapof the mask in advance of the exposure and controlling the mask positionin at least one of Z, Rx and Ry during the exposure. The height map canbe generated in a similar manner to that described above (i.e. off-axislevelling of the mask at a measurement station); however, it may also begenerated with the mask at the exposure station, which may obviate theneed to relate the height map to a physical reference surface. Thecalculation of the optimum position(s) of the mask during the exposureor exposure scan (the exposure path) can be equivalent to that describedabove, but it may also be a coupled optimization of wafer and maskexposure paths. However, for a mask, it may be advantageous to placegreater weight in the optimization calculations on tilt deviations,since these will have a greater effect on the position at the wafer.

It should be explicitly noted that a lithographic projection apparatusaccording to an embodiment of the current invention may contain two (ormore) substrate tables and/or two (or more) mask tables. In such ascenario, it may be possible for a first substrate on a first substratetable to be undergoing height-mapping at the measurement station while asecond substrate on a second substrate table is concurrently undergoingexposure at the exposure station; and similarly in the case of multiplemask tables. Such a construction can greatly increase throughput.

It should also be explicitly noted that embodiments of the currentinvention can be applied to substrate leveling alone, to mask levelingalone, or to a combination of substrate leveling and mask leveling.

Embodiments of the present invention may relate to lithographicapparatus using catoptrics in an optical sensor system and to devicemanufacturing methods for such apparatus. Embodiments of the presentinvention may also relate to optical sensing (for example, of thesubstrate and/or mask) in lithographic apparatus. A lithographicapparatus according to one embodiment of the invention includes anillumination system for providing a projection beam of radiation; asupport structure for supporting patterning structure, the patterningstructure serving to impart the projection beam with a pattern in itscross-section; a substrate table for holding a substrate; a projectionsystem for projecting the patterned beam onto a target portion of thesubstrate; and the lithographic apparatus further comprising at leastone of an optical detector and an optical sensor for determining aposition of the target portion in the patterned beam of radiation.

A lithographic apparatus according to a further embodiment of theinvention includes an illumination system for providing a projectionbeam of radiation; a support structure for supporting patterningstructure, the patterning structure serving to impart the projectionbeam with a pattern in its cross-section; a substrate table for holdinga substrate; and a projection system for projecting the patterned beamonto a target portion of the substrate; and the lithographic apparatusfurther comprising at least one of an optical detector and an opticalsensor for determining a position of the target portion in the patternedbeam of radiation, wherein the at least one of an optical detector andan optical sensor comprises at least one catoptrical system (RE), saidat least one catoptrical system (RE) being arranged to have an imagingfunction of at least one dioptrical element.

A lithographic apparatus and a device manufacturing method according toat least some embodiments of the present invention may be used to allowoperation of the optical system in any ambient, such as an inert(nitrogen) ambient, a liquid, or vacuum with limited or no need ofproviding a correction during testing. The optical system can be set-upand calibrated during installation or maintenance, without considerationof the ambient conditions during the manufacturing cycle of thelithographic apparatus.

A lithographic projection apparatus according to a further embodiment ofthe invention includes an illumination system for providing a projectionbeam of radiation; a support structure for supporting patterningstructure, the patterning structure serving to impart the projectionbeam with a pattern in its cross-section; a substrate table for holdinga substrate; and a projection system for projecting the patterned beamonto a target portion of the substrate; the lithographic apparatus beingprovided with a level sensor for determining the height of the wafersurface, wherein the level sensor comprises a system for detecting animage projected onto the substrate, which system comprises essentiallyor only reflecting optical elements.

A lithographic projection apparatus according to a further embodiment ofthe invention includes an illumination system for providing a projectionbeam of radiation; a support structure for supporting patterningstructure, the patterning structure serving to impart the projectionbeam with a pattern in its cross-section; a substrate table for holdinga substrate; and a projection system for projecting the patterned beamonto a target portion of the substrate; the lithographic apparatus beingprovided with a level sensor for determining the height of the wafersurface, wherein the level sensor comprises a system for detecting animage projected onto the substrate, which system comprises essentiallyor only reflecting optical elements.

A device manufacturing method according to a further embodiment of theinvention includes providing a substrate; providing a projection beam ofradiation using an illumination system; using patterning structure toimpart the projection beam with a pattern in its cross-section;projecting the patterned beam of radiation onto a target portion of thesubstrate; and using at least one of an optical detection action and anoptical sensing action to position the target portion in the patternedbeam of radiation, wherein the at least one of an optical detectionaction and an optical sensing action comprises using at least onecatoptrical system (RE) which comprises only reflective elements, saidat least one catoptrical system (RE) being arranged to have an imagingfunction of at least one dioptrical element.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention as claimed may be practicedotherwise than as described. For example, embodiments of the method mayalso include one or more computers, processors, and/or processing units(e.g. arrays of logic elements) configured to control an apparatus toperform a method as described herein, or a data storage medium (e.g. amagnetic or optical disk or semiconductor memory such as ROM, RAM, orflash RAM) configured to include instructions (e.g. executable by anarray of logic elements) describing such a method. It is explicitlynoted that the description of these embodiments is not intended to limitthe invention as claimed.

1. A lithographic apparatus comprising: a projection system configuredto project a patterned beam of radiation onto a target portion of asubstrate; and a measurement system having at least one catoptricalsystem, wherein the catoptrical system includes two or more reflectiveelements and wherein said measurement system is configured to determinea position of the target portion of the substrate relative to a focusplane of the projection system using a measurement beam having anoptical path that traverses the at least one catoptrical system, whereinsaid at least one catoptrical system is configured to perform an imagingfunction of at least one dioptrical element.
 2. The lithographicapparatus according to claim 1, wherein said measurement system includesat least one of an optical sensor and an optical detector, and whereinsaid at least one of the optical sensor and the optical detectorincludes said at least one catoptrical system.
 3. The lithographicapparatus according to claim 1, wherein said measurement system isconfigured to determine the position of the target portion of thesubstrate in a vacuum ambient.
 4. The lithographic apparatus accordingto claim 1, wherein said measurement system is configured to determinethe position of the target portion of the substrate in a liquid ambient.5. The lithographic apparatus according to claim 1, wherein saidmeasurement system is configured to determine the position of the targetportion of the substrate in an ambient substantially comprising at leastone among nitrogen, helium, and argon.
 6. A lithographic apparatuscomprising: a projection system configured to project a patterned beamof radiation onto a target portion of a substrate; and a measurementsystem having at least one catoptrical system, wherein the catoptricalsystem includes two or more reflective elements and wherein saidmeasurement system is configured to determine a position of the targetportion of the substrate relative to a focus plane of the projectionsystem using a measurement beam having an optical path that traversesthe at least one catoptrical system, wherein said at least onecatoptrical system is configured to perform an imaging function of adioptrical arrangement that includes at least one among a convex lens, aconcave lens, a planoconvex lens, and a planoconcave lens.
 7. Alithographic apparatus comprising: a projection system configured toproject a patterned beam of radiation onto a target portion of asubstrate; and a measurement system having at least one catoptricalsystem, wherein the catoptrical system includes two or more reflectiveelements and wherein said measurement system is configured to determinea position of the target portion of the substrate relative to a focusplane of the projection system using a measurement beam having anoptical path that traverses the at least one catoptrical system, whereinthe at least one catoptrical system includes at least one among aconcave mirror and a convex mirror.
 8. A lithographic projectionapparatus comprising: a projection system configured to project apatterned beam of radiation onto a target portion of a substrate; and ameasurement system configured to determine a position of the targetportion of the substrate relative to a focus plane of the projectionsystem using at least one among an optical sensing operation and anoptical detecting operation, wherein the measurement system isconfigured to determine the position via a measurement beam thattraverses an optical path that includes at least one catoptrical system,wherein the catoptrical system include two or more reflective elements,wherein the catoptrical system is arranged to have an imaging functionof at least one dioptrical element.
 9. The lithographic projectionapparatus according to claim 8, wherein said measurement system isconfigured to determine the position of the target portion of thesubstrate in a vacuum ambient.
 10. A lithographic projection apparatuscomprising: a projection system configured to project a patterned beamof radiation onto a target portion of a substrate; and a measurementsystem configured to determine a position of the target portion of thesubstrate relative to a focus plane of the projection system using atleast one among an optical sensing operation and an optical detectingoperation, wherein the measurement system is configured to determine theposition via a measurement beam that traverses an optical path thatincludes at least one catoptrical system, wherein the catoptrical systeminclude two or more reflective elements, wherein said at least onecatoptrical system is configured to perform an imaging function of adioptrical arrangement that includes at least one among a convex lens, aconcave lens, a planoconvex lens, and a planoconcave lens.
 11. Alithographic projection apparatus comprising: a projection systemconfigured to project a patterned beam of radiation onto a targetportion of a substrate; and a measurement system configured to determinea position of the target portion of the substrate relative to a focusplane of the projection system using at least one among an opticalsensing operation and an optical detecting operation, wherein themeasurement system is configured to determine the position via ameasurement beam that traverses an optical path that includes at leastone catoptrical system, wherein the catoptrical system include two ormore reflective elements, wherein the at least one catoptrical systemincludes at least one among a concave mirror and a convex mirror.
 12. Alithographic apparatus comprising: a projection system configured toproject a patterned beam of radiation onto a target portion of a subsubstrate; and a level sensor that includes at least one catoptricalsystem, wherein the catoptrical system includes two or more reflectiveelements, wherein the level sensor is configured to determine a heightof a surface of the substrate using a beam that traverses an opticalpath that includes the at least one catoptrical system, and wherein theat least one catoptrical system is configured to perform an imagingfunction of at least one dioptrical element.
 13. The lithographicapparatus according to claim 12, wherein the level sensor comprises asystem that includes the at least one catoptrical system and isconfigured to project an image onto the substrate.
 14. The lithographicapparatus according to claim 12, wherein the level sensor comprises asystem that includes the at least one catoptrical system and isconfigured to detect an image projected onto the substrate.
 15. Thelithographic apparatus according to claim 12, wherein the optical pathof the level sensor includes no refractive elements to guide the beam.16. The lithographic apparatus according to claim 15, wherein the levelsensor is configured to determine the height of the surface of thesubstrate in a vacuum ambient.
 17. A device manufacturing method, saidmethod comprising: projecting a patterned beam of radiation onto atarget portion of a substrate; and using at least one of an opticaldetection action and an optical sensing action to position the targetportion of the substrate relative to a focus position of the patternedbeam of radiation, wherein the at least one optical detection action andthe optical sensing action is performed using a beam that traverses anoptical path that includes at least one catoptrical system, wherein thecatoptrical system includes two or more reflective elements, wherein theat least one catoptrical system is configured to perform an imagingfunction of at least one dioptrical element.
 18. The devicemanufacturing method according to claim 17, wherein said using at leastone of the optical detection action and the optical sensing action toposition the target portion includes determining a height of a surfaceof the substrate.
 19. The device manufacturing method according to claim18, further comprising adjusting the position of the target portion ofthe substrate based on the determined height.
 20. The devicemanufacturing method according to claim 17, wherein the target portionis sensitive to radiation of the patterned beam.
 21. The devicemanufacturing method according to claim 17, wherein said using at leastone of the optical detection action and the optical sensing action isperformed in a vacuum ambient.
 22. The device manufacturing methodaccording to claim 17, wherein said using at least one of the opticaldetection action and the optical sensing action is performed in a liquidambient.
 23. A device manufactured according to the method according toclaim
 17. 24. A device manufacturing method, said method comprising:projecting a patterned beam of radiation onto a target portion of asubstrate; and using a measurement system to position the target portionrelative to a focus position of the patterned beam of radiation, whereinthe measurement system uses a beam that traverses an optical path thatincludes at least one catoptrical system and wherein the catoptricalsystem includes two or more reflective elements, and wherein the atleast one catoptrical system is configured to perform an imagingfunction of at least one dioptrical element.
 25. The devicemanufacturing method according to claim 24, said method furthercomprising performing a calibration of the measurement system in a firstambient and positioning the target portion in a second ambient that isdifferent from the first ambient.
 26. The device manufacturing methodaccording to claim 25, wherein one of the first and second ambient is avacuum ambient.
 27. The device manufacturing method according to claim25, wherein performing the calibration of the measurement systemincludes using the measurement system to determine a height of a surfaceof the substrate.
 28. The device manufacturing method according to claim27, wherein said using the measurement system to position the targetportion includes determining a height of the target portion.
 29. Alithographic apparatus comprising: a projection system configured toproject a patterned beam of radiation onto a target portion of asubstrate; and a measurement system having at least two catoptricalsystems, wherein the at least two catoptrical systems include two ormore reflective elements, wherein said measurement system is configuredto determine a position of the target portion of the substrate relativeto a focus plane of the projection system using a measurement beamhaving an optical path that traverses the at least two catoptricalsystems, and wherein at least one of said at least two catoptricalsystems is configured to perform an imaging function of at least onedioptrical element.
 30. A lithographic apparatus comprising: aprojection system configured to project a patterned beam of radiationonto a target portion of a substrate; and a measurement system having atleast two catoptrical systems, wherein at least one of the twocatoptrical systems includes two or more reflective elements and whereinsaid measurement system is configured to determine a position of thetarget portion of the substrate relative to a focus plane of theprojection system using a measurement beam having an optical path thattraverses the at least two catoptrical systems, and wherein the at leastone of the two catoptrical systems is configured to perform an imagingfunction of at least one dioptrical element.